1 实验方法
将Q235钢加工成20.0 mm长的钢丝 (
为了研究南海热带环境下藤壶自然附着对金属的腐蚀行为影响，选择在藤壶生长繁殖最旺盛的夏季，于湛江调顺岛进行挂样实验。挂样地点水深约为15 m，潮差较大，海水电导率为36000 μS·cm-1，盐度为24.5，pH为7.86。将阵列电极牢牢固定在浮岛上，确保其工作面完全浸泡在海水中。
在实验的15和30 d取回样品，使用Nikon D800E单反相机记录电极表面宏观腐蚀形貌，并利用CS350电化学工作站进行电化学测试。线性极化和交流阻抗测试以2 cm
2 结果与讨论
2.1 碳钢在海水浸泡15和30 d的腐蚀规律
图1
图1
Q235钢电极浸泡不同周期的电偶电流和偶接电位分布图及其对应表面形貌图
Fig.1
Galvanic current and coupling potential distribution maps (a, b, d, e) and surface topography maps (c, f) for 15 d (a-c) and 30 d (d-f)
浸泡15 d，根据图1c可知，金属表面生成黄褐色腐蚀产物，表面有少数小藤壶附着。对距藤壶不同距离的4个微电极进行线性极化测试，极化曲线见图2a。图2b为距藤壶不同距离微电极的Icorr和Ecorr对比图，B和D微电极金属表面都直接暴露在海水中，二者Icorr相近，数值约为10 μA·cm-2，而B比D微电极更靠近藤壶附着位置，B微电极的Ecorr 更低，两者相差38 mV。A和C微电极表面都有腐蚀产物堆积，二者Icorr均小于仅有少量腐蚀产物覆盖的B、D微电极，腐蚀产物一定程度上会抑制金属腐蚀，而C微电极更靠近藤壶附着位置，C微电极Ecorr低于A微电极的Ecorr。由此推断藤壶附着会造成周围区域金属的腐蚀电位降低。
图2
图2
藤壶附着及周围区域的线性极化曲线
Fig.2
Linear polarization plots of barnacle attachment and surrounding area on 15 d (a, b) and 30 d (c, d)
浸泡30 d，根据图1f可知，电极表面50%以上的区域被藤壶覆盖，a微电极表面被藤壶覆盖，其Icorr为1.5 μA·cm-2，Ecorr为-0.7207 V，皆明显低于周围区域，且随距藤壶距离的增加，微电极Icorr和Ecorr皆呈增大趋势。b微电极表面被较厚腐蚀产物覆盖，其Icorr为3.9 μA·cm-2，明显低于仅有少量腐蚀产物的c微电极的Icorr，高于被藤壶覆盖的a微电极的Icorr，由此推断，虽然大型污损生物层和锈层下均含有大量的异养菌、铁细菌和中性硫氧化菌[13, 14]，但藤壶石灰质底壳比碳钢表面腐蚀产物层更致密，底壳抑制营养物质传输和氧气扩散作用更强，使得下方细菌活性减弱，a微电极Icorr低于b微电极Icorr。同时，更为致密的藤壶底壳抑制溶解氧的传输，造成壳下微生物群落逐渐转变为厌氧菌和兼性厌氧菌，细菌通过分解有机体，产生酸性和有机活性物质[15~18]，导致pH值下降、Ecorr低于未被藤壶附着的微电极。藤壶分泌的石灰质或摄食、代谢过程中分泌的有机物，使碳钢腐蚀产物的粘附性增加，结合更为牢固。因此，临近藤壶区域的电极表面腐蚀产物多于远离藤壶区域的腐蚀产物，腐蚀产物的累积继而影响物质的传输。
2.2 藤壶对金属腐蚀行为的影响
为了进一步阐明藤壶对金属表面的电化学行为的影响，选取图1f中有藤壶附着的a微电极、无藤壶附着的c微电极和表面有残余壳体的d微电极进行电化学阻抗测试。图3a和b为对应的Nyquist图和Bode图。从Bode曲线中可知，在|Z|0.01 Hz 频率下有藤壶附着微电极阻抗模值最大，为378.3 Ω·cm2，而无藤壶附着的微电极阻抗模值为237.8 Ω·cm2，有残余壳体的电极阻抗模值低于无藤壶附着的电极，仅为156.2 Ω·cm2。活体藤壶的阻隔性高于碳钢腐蚀产物的阻隔性，碳钢腐蚀产物的阻隔性高于藤壶残余壳体的阻隔性。藤壶附着对金属腐蚀起抑制作用，藤壶底壳主要成分为碳酸钙，导电性差，同时藤壶分泌的藤壶胶具有较大的内聚强度[19, 20]，能将基体表面和其钙质底盘通过几微米的胶层连接起来，并通过粘腺导管网络系统运输液态胶到附着部位，液态胶又能通过流动和扩散作用，使基底与基体间的接触更为紧密[21,22]，阻止海水直接与基体发生接触，限制溶解氧等成分的传输。藤壶因种内竞争导致脱落后，残余底壳中的有机质被细菌和微生物降解后其致密性大幅下降，其阻隔性大幅下降。
图3
图3
电化学阻抗图以及阻抗等效电路模型
Fig.3
Nyquist (a) and Bode (b) plot represents the fitted curve of the three samples, equivalent circuit diagram of attached by part of balanite or not attached by barnacle (c), and attached by barnacles (d)
利用Zview软件对阻抗谱进行拟合，图3c为藤壶脱落和无藤壶附着时的拟合等效电路，图3d为有藤壶附着时的拟合等效电路。为了消除非理想电容对拟合结果造成的影响，使用常相位角元件CPE，CPEf、CPEdl分别代表藤壶/碳钢之间的电容和双电层电容，Rs、Rf、Rct分别代表溶液电阻、藤壶/碳钢之间的孔隙或锈蚀电阻和电荷转移电阻，Warburg阻抗用于表明碳钢受扩散控制的腐蚀反应。表1为等效电路图中各等效电子元件的拟合数值。从表中可得，有藤壶附着区域Rct为19.81 Ω·cm2，无藤壶附着区域Rct为15.26 Ω·cm2，残余壳体区域Rct仅为1.629 Ω·cm2，因Rct与金属腐蚀速率负相关，可知藤壶附着下a微电极的腐蚀速率小于未被藤壶附着的c微电极，该结果与图2d极化曲线求得的瞬时腐蚀速率具有较好的一致性。因藤壶脱落区域残余的钙质底壳逐渐被细菌分解，金属重新暴露在海水中，使得d微电极的Rct远小于a、c微电极。a微电极的弥散指数CPEdl-P大于c、d微电极，电极表面被完整的生物底壳覆盖，耐蚀性更好。
表1 等效电路拟合值
Table 1
| Sample | Rs Ω·cm2 | CPEdl | RctΩ·cm2 | W | CPEf | Rf Ω·cm2 | ||||
|---|---|---|---|---|---|---|---|---|---|---|
CPEdl-T S·sec n ·cm2 | CPEdl-P | R Ω·cm2 | T S0.5 | P | CPEf-T S·sec n ·cm2 | CPEf-P | ||||
| Attached by barnacles | 4.499 | 0.01280 | 0.72397 | 19.81 | 42.91 | 0.4261 | 0.4313 | 0.00077 | 0.24652 | 26.91 |
| Not attached by barnacles | 10.59 | 0.00483 | 0.51994 | 15.26 | 95.96 | 2.855 | 0.3167 | - | - | - |
| Attached by part of balanite | 10.85 | 0.00072 | 0.65406 | 1.629 | 34.99 | 2.742 | 0.4151 | - | - | - |
3 结论
(1) 藤壶附着造成碳钢的非均匀腐蚀，从浸泡15 d有小藤壶附着到30 d藤壶逐渐长大，碳钢微电极间偶接电位持续降低，电偶电流增加，最大偶接电位差从25 mV下降为9 mV，最大阳极区电偶电流从32 μA·cm-2降低为18.6 μA·cm-2，最大阴极区电偶电流增加到41.6 μA·cm-2。
(2) 随距藤壶附着位置距离的增加，碳钢腐蚀电位逐渐升高，腐蚀电流逐渐加大。藤壶底壳下碳钢的腐蚀电位比距藤壶附着点较远区域的腐蚀电位低25 mV，藤壶底壳下碳钢的腐蚀电流约为距藤壶附着点较远区域的腐蚀电流的21%。
(3) 活体藤壶壳体为碳酸钙和有机质的复合材料，具有较好的阻隔性，其抑制溶解氧的传输造成藤壶活体附着下碳钢的低腐蚀电位、低腐蚀电流的“双低”现象；藤壶脱落后，经微生物分解有机质后，残存壳体阻隔性大为降低，其阻隔性低于碳钢腐蚀产物阻隔性，促使碳钢快速腐蚀。藤壶生长过程中分泌的有机物与藤壶附着点周围碳钢的腐蚀产物混合，增大了腐蚀产物的粘附强度，促进腐蚀产物的累积，增大腐蚀产物的阻隔性。海洋环境中，藤壶对碳钢的附着会加剧碳钢时空二维的非均匀腐蚀。
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